The present invention relates to an hydraulic brake system, in particular for anti-lock control.
A type of hydraulic brake system is known from German patent application P 40 02 730 comprising a first and a second booster piston accommodated in a braking pressure generator housing which both can be acted upon by the pressure of an auxiliary pressure source via a braking pressure control valve arranged in the braking pressure generator housing. In dependence on a push rod piston abutting on the first auxiliary pressure piston, the braking pressure control valve establishes a hydraulic connection either between the auxiliary pressure source and the first booster piston, in order to supply in a power-assisted fashion pressure fluid for the wheel brakes to the working pistons of the tandem master cylinder connected downstream of the booster piston, or it connects the booster piston with an unpressurized supply reservoir by means of a connection governed by the braking pressure control valve. Further, a pressure-reducing valve is interposed between the auxiliary pressure source and the hydraulic booster in order to limit the system pressure.
However, the described system has the disadvantage that relatively high pressures may occur in the tandem master cylinder in the event of anti-lock control since upon start-up of an auxiliary pressure pump required for the anti-lock control, the said pump will counteract the tandem master cylinder pressure, or the admissible stress limit of the system component parts will be reached upon failure of the front-axle brake circuit in consequence of the hydraulic transmission ratio and the boosting ratio of the brake power booster that is defined by construction.
Therefore, it is the object of the present invention to limit the pressure level prevailing in the braking pressure generator to an admissible extent if brake circuit failure occurs and/or if the described brake system is used for anti-lock control purposes.